The present invention relates to a magnetic torsion meter for measuring torques by means of a torsion shaft and which is particularly adapted for measuring large size shaft torsion, such as marine screws propeller shafts, turbine propeller shafts, grinder propeller shafts, etc . . . , subject to a torque effect.
Torsion meters generally use a torsion bar having its extremities are connected respectively to a shaft end providing an engine torque and to a shaft end presenting a load moment. The torsion bar which can rotate relative to an exterior box having two annular gears and their respective stationary coils, comprises two toothed disks mounted on both extremities of the torsion bar. Whenever the disk teeth pass in front of the gear teeth, a variation of the magnetic flux and a signal modulation in the coils are produced correlatively and their respective phase shift can then be measured with a phase indicator. However, such a device requires the use of complicated electronic circuits necessary to the phase shift synchronization. In addition this measuring method is not suited to the study of transient phenomenon and is not adapted to measuring when stopped.
Hence, French patent FR 2 689 633 proposed a magnetic torsion meter made of an angular comparator comprising first and second toothed rotors made of ferromagnetic material. The first and second rotors are respectively connected to the first and second extremities of a rotating shaft, the teeth of each rotor facing each other and defining a gap. An exterior magnetic circuit with a constant reluctance is a static circuit made of ferromagnetic material with its first and second extremities respectively facing non-toothed cylindrical parts of the first and second toothed rotors while respectively defining between them first and second transfer gaps. In such a torsion meter, a Hall effect sensor is installed in a measuring gap defined in the exterior magnetic circuit. An excitation source creates a flux. The sensor delivers an amplification electric signal proportional to the reluctance variation in the measuring gap and therefore to the torque applied to the rotating shaft.
However, such a device does not allow for carrying out an absolute torque measurement as there is no zero reference. This is an important feature of this device.
It will be noted that an electronic correction of the measuring signal, which would make it possible to display zero for a nil torque, is not able to cover the whole range of variation of the perturbing parameters of mechanical or thermal origin.
Moreover it would require an absolute stability of the power supply source of the Hall effect sensor.
It would be possible to imagine an improvement of the device mentioned in patent FR 2 689 633, in order to obtain an absolute measurement of the torque, by attaching two similar devices and mounting them head to tail along the torsion shaft, these equal and opposite device measuring signals counteracting each other, the common face being used as a reference.
This solution is not entirely satisfactory as the two devices, despite their similar design, would necessarily have imperfections and their Hall effect sensors would have different laws of variation, and then still resulting in an impossible compensation all throughout the signal variation range.
In conclusion, the device of French patent FR 2 689 633, whatever the improvements are, does not make the absolute measurement of the torsion and torque possible.